Structural bonding tapes are useful for bonding one or more substrates to one another. Many structural bonding tapes fall into one of two groups: (1) heat-curable structural bonding tapes, and (2) ultraviolet (UV) light-activatable structural bonding tapes. As suggested by the class name, heat-curable structural bonding tapes require heat to cure the adhesive composition of the tape. The UVi structural bonding tapes contain an adhesive composition that will begin to cure when exposed to UV light, but does not require heat for curing. Heat may be used to accelerate the rate of cure for UVi structural bonding tapes.
Typically, a length of structural bonding tape or a die cut piece of tape is removed from a roll, and attached to a first substrate using finger pressure. In the case of UVi structural bonding tapes, the structural bonding tape can be exposed to actinic radiation, such as UV. Then, a second substrate is brought into contact with the exposed surface of the structural bonding tape, and pressure is applied to the substrates for a period of time. Heat-curable structural bonding tapes, and optionally UVi structural bonding tapes, are then exposed to heat and the assembly is then allowed to cool. The result is a bonded article.
Further, conventional Uvi structural bonding tape formulations possess high cold flow properties. Cold flow is a measure of the creep behavior of a material at non-elevated temperatures. Many conventional UVi tape materials possess undesirably high cold flow properties, which result in significant material flow under roll winding tensions and stacking weight. As a result, these materials may require cold storage and/or special packaging to maintain dimensionally stable rolls and die cut parts.
In addition, many current structural adhesive compositions require heat to cure the adhesive composition. In a process of making bonded articles, the bonded article must be subjected to a heating step in order to cure the adhesive composition as well as a cooling step in order to allow for further handling and/or packaging of the bonded article. From a processing standpoint, a process of making bonded articles without a heating step would be highly desirable.
What is needed in the art is a structural adhesive or bonding tape formed from an adhesive composition formulation, which has low temperature bonding performance and/or may be used to bonding different substrates having different coefficients of thermal expansion. In addition, what is needed in the art is a UVi structural bonding tape having a tape construction, which allows the tape to be light-activatable from one side of the tape. Typical substrates relevant to UVi structural bonding tape are not transparent to UV light and therefore prevent UV initiation after the bond is fully assembled.